1. Field of the Invention
The invention relates to processes and systems for purifying boron trichloride. In particular, the invention relates to processes and systems or apparatus which remove several critical impurities of boron trichloride to produce a highly purified final product required for some of its more stringent applications.
2. Related Art
Boron trichloride (also referred to herein as xe2x80x9cBCl3 xe2x80x9d) is a highly reactive compound packaged as a liquid under its own vapor pressure of 1.3 bar (130 kPa) absolute at 21xc2x0 C. that has numerous diverse applications. It is used predominantly as a source of boron in a variety of manufacturing processes. For example, in the manufacturing of structural materials, boron trichloride is the precursor for chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) of boron filaments used to reinforce high performance composite materials. BCl3 is also used as a CVD precursor in the boron doping of optical fibers, scratch resistant coatings, and semiconductors. Some of the non-CVD applications of BCl3 are reactive ion etching of semiconductor integrated circuits and refining of metal alloys. In metallurgical applications, it is used to remove oxides, carbides, and nitrides from molten metals. In particular, BCl3 is used to refine aluminum and its alloys to improve tensile strength.
Two of the most stringent applications for high purity BCL3 involve semiconductor and optical fiber manufacturing. In these industries the specified impurity levels in BCl3 must be of the order of 1 ppm or less in order to maintain product quality. In fact, the impurities in most commercially available BCl3 are often present at levels over two orders of magnitude beyond acceptable levels for these processes such as, for example, air, CO2, HCl, Cl2, and COCl2 (xe2x80x9cphosgenexe2x80x9d). Furthermore, in these particular applications, any oxygen or oxygen containing impurities (such as phosgene) in the BCl3 are especially detrimental to the manufacturing process due to the formation of certain oxide compounds. Another class of detrimental impurities in BCl3 for these processes are metal containing impurities.
Geographically, BCl3 is produced almost entirely in the United States. As of 1995, as much as 220 metric tons has been consumed in the United States where about 30% has gone into the production of boron reinforcement filaments, the remaining split primarily among semiconductor etching, Friedel-Crafts catalysis reactions, and intermediate use in pharmaceuticals. In comparison, Japan consumes 70 metric tons which was all imported from the United States. In Japan, BCl3 is used primarily in semiconductor etching and manufacture of crucibles for silicon ingots. Western European countries consumed only about 5 metric tons. (Chemical Economics Handbook, October, 1996.) The source cost of BCl3 varies considerably per pound depending upon purity grade and supplier. There is a strong incentive to purchase BCl3 domestically at a low cost and purify the material to stringent semiconductor purity requirements of technically 1 ppm or less for the light impurities.
After extensively searching the literature and patents, there appears to be no production process technology to have been described or patented regarding how to efficiently remove various impurities from boron trichloride by an integrated purification process technology comprising several different functional chemical processes which are connected sequentially and various impurities associated with boron trichloride are removed sequentially and continuously.
The removal of some impurities in BCl3 has been disclosed previously. In particular, most publications have focused on how to remove phosgene from boron trichloride. This is because phosgene has similar vapor pressure to BCl3 and hence becomes difficult to remove by simple distillation. The previous methods for phosgene removal from BCl3 include electrical discharge, laser pyrolysis, fractional distillation, UV photolysis, and redox chemistry.
Although the individual methods aforementioned had indicated to be able to reduce phosgene content in boron trichloride to a certain degree, these methods do have their drawbacks. For instance, the use of electrical discharge and laser pyrolysis is difficult to implement on a larger industrial scale without extensive equipment and capital costs, and therefore, the economics are not feasible. UV photolysis lacks effectiveness for phosgene removal to very low ppm levels. Further, the similarity of physical properties of phosgene and boron trichloride makes phase separation by distillation and differential surface adsorption difficult to implement in a practical manner. It is also known to use selective chemistry to remove phosgene from BCl3. In these methods phosgene in the BCl3 is allowed to oxidize molten metals such as mercury, copper, and titanium to form the corresponding metal chlorides and carbon monoxide. Although effective in removing phosgene, this approach presents problems with metal contamination, which is particularly difficult due to the volatility of metal chlorides.
In view of all the drawbacks aforementioned, the preferred process of removing phosgene is by thermal decomposition via a catalyst with a specified elevated temperature. For example, the phosgene decomposition on a preferably metal free carbonaceous catalyst was described by two earlier publications. However, in each of these two cases, other troublesome impurities were generated (chlorine in one case, and hydrogen chloride in the other) which require independent purification steps.
Another problem with known BCl3 purification methods is the need to resort to vacuum generating devices or thermal heating of source material and associated handling systems to improve the rate of vapor transport through packed beds of adsorbents or catalytic materials. In known BCl3 purification methods using packed beds such as the case of carbonaceous catalysts, there are significant pressure drops associated with packed beds when high volumetric flow rates are employed and good surface contact required. For many gases, this is not a problem. But, when it comes to BCl3, material transport through such pressure drops becomes significantly hindered due to the BCl3 liquid having only a 1.3 bar vapor pressure at ambient temperature. Thus, maintaining reasonable flow rates through such devices requires some auxiliary means of promoting flow. Conventionally, flow throughput can be advanced by either increasing upstream pressure or decreasing downstream pressure. Increasing upstream pressure can be done using commonly known techniques of gravimetric feeding, mechanical pumping, or thermal heating of source material. However, in the specific case of producing high purity corrosive gases like BCl3, the reactive nature of BCl3 makes the mechanical devices undesirable requiring high maintenance and excessive costs while providing low reliability and the increased likelihood of contamination of the BCl3 by metallic impurities. Gravimetric feeding (in other words, elevating source material relative to the rest of the system) effectively promotes flow as only 2 meter height provides almost 1 bar additional upstream pressure. However, this approach still suffers from the intolerable feature of requiring material transport through the system as entirely liquid phase instead of vapor phase. As a consequence of liquid phase present in the system, excessive contamination of BCl3 by metallic impurities can occur from enhanced liquid phase corrosion mechanisms thereby degrading product purity with detrimental metallic impurities.
One known method of increasing upstream pressure with vapor condensation downstream is to heat the source material and all associated gas handling components to an isothermal temperature. The method is feasible but requires careful temperature control to assure uniform temperature throughout the system. Although feasible, this technique becomes difficult to implement in practice especially for high capacity industrial production.
Resorting to decreasing downstream pressure has its difficulties also. The simplest approach of mechanical pumping suffers from the same problems as in the upstream case. The use of simple low temperature condensation of BCl3 downstream prevents the problems of mechanical pumping but will lead to accumulation of metallic impurities in the final product collected hence degrading purity.
In the processes of the present invention, phosgene removal is performed by the preferred thermal decomposition route in a manner in which the decomposition impurities are preferably continuously removed. In accordance with the present invention, low temperature condensation is utilized along with secondary inert gas stream such as He, N2, or Ar. In this technique, as disclosed in further detail herein below, the BCl3 material is carried through the defined purification system alone with a secondary inert gas stream. The presence of such a gas stream having higher vapor pressure allows the overall system to be operated at higher pressures than that provided from BCl3 vapor pressure alone. This is preferably performed most simply by bubbling the inert gas through the liquid BCl3 and flowing the mixed gas stream through the system, after which the inert gas is easily separated from the purified BCl3 product collected.
A first aspect of the invention is a process of producing a BCl3 vapor stream containing an inert gas selected from the group consisting of helium, argon, krypton, neon, xenon, or mixtures of one or more of these, from a lower purity BCl3 source, the BCl3/inert gas vapor stream having less than 10 ppm chlorine, less than 10 ppm phosgene, and less than 10 ppm each of light impurities including, but not limited to, nitrogen, oxygen, carbon dioxide, carbon monoxide, and hydrocarbons such as methane, and less than 10 ppm of nonvolatile metal containing species. In one embodiment, using helium as the inert gas, the process comprises injecting helium into a container of a lower purity BCl3 source having phosgene impurity to produce a vapor stream comprising BCl3, helium, and phosgene; decomposing a major portion of the phosgene in the BCl3, helium, phosgene vapor stream by heating the vapor stream to a first temperature, in the presence of a first material, to decompose substantially all the phosgene to carbon monoxide and chlorine, to form a first intermediate vapor stream comprising BCl3, helium, carbon monoxide, and less than 10 ppm phosgene; and adsorbing a major portion of the chlorine in the first intermediate vapor stream at a temperature lower than the first temperature using a second material, thereby producing the BCl3/helium vapor stream having less than less than 10 ppm chlorine, less than 10 ppm phosgene, and less than 10 ppm each of the light impurities. In preferred processes of the invention, the first and second materials are substantially the same.
A preferred process embodiment in accordance with this aspect of the invention is wherein the heating step comprises preheating the vapor stream comprising BCl3, helium, and phosgene prior to the vapor stream comprising BCl3, helium, phosgene contacting the first material, which promotes phosgene decomposition.
A particularly preferred process embodiment in accordance with this aspect of the invention is wherein the preheating comprises heat exchanging the first intermediate vapor stream with the vapor stream comprising BCl3, helium, and phosgene.
Preferably, the phosgene decomposition step occurs in the presence of a catalyst, the catalyst comprising materials selected from the group consisting of carbon-based materials, alumina-based materials, silica-based materials, and mixtures thereof. Preferably, if carbon is used, it is selected from the group consisting of naturally occurring carbon, carbon molecular sieve, or other synthetic carbonaceous material. Alternatively, phosgene decomposition can be implemented in the processes of the invention with other reactive elements such as boron, silicon, and various metals such as titanium or zinc, as described in U.S. Pat. Nos. 3,037,337; 3,043,665; and 3,207,581; however, such elements are not catalytic as they are consumed in the process, and are thus subject to depletion, thus they are not therefore the preferred materials for the phosgene decomposition step.
In accordance with this aspect of the invention, the inert gas functions to increase pressure of the vapor stream comprising BCl3, inert gas, and phosgene to a pressure substantially higher than the vapor pressure of the lower purity BCl3.
Preferably, the phosgene decomposition step occurs at a temperature greater than about 200xc2x0 C., and the adsorption of chlorine step preferably occurs at a temperature lower than about 50xc2x0 C., although some chlorine will be adsorbed on the first material at a higher temperature in the phosgene decomposition step.
Furthermore, the chlorine adsorption step preferably comprises using a bed of adsorbent until loaded, removing the bed of adsorbent, heating the removed bed of adsorbent, and reinstalling the bed. More preferably, a second chlorine adsorption bed of same or different adsorbent could be utilized while the first is regenerating, in order to maintain continuity of the process. Alternatively, but less preferable, is the use of one bed of chlorine adsorbent with the appropriate valve configuration to allow isolation from the process and conduit connection to a regeneration system, be it via heated purge or vacuum induced desorption.
A second aspect in accordance with the invention is a process for producing an ultra-pure BCl3 condensed phase from a vapor phase comprising impure BCl3. The process comprises condensing a first vapor stream in a condenser, the first vapor comprising a major portion of BCl3 and a minor portion of HCl, light impurities, and a first inert gas selected from the group consisting of helium, argon, krypton, neon, xenon, and mixtures thereof, to form a first condensed phase comprising BCl3 and a second vapor comprising the first inert gas, BCl3, and light impurities; routing the second vapor stream to a secondary condenser, at a lower temperature, thus forming a gaseous stream containing HCl, light impurities, and the first inert gas and a second condensed phase comprising BCl3; and routing the first condensed phase to a stripper, or using the condenser itself at a more optimal temperature, wherein a second inert gas (the same as or different from the first) is used to strip molecules having vapor pressure greater than BCl3 from the first condensed phase to produce a higher purity first condensed phase having less than 50 ppm hydrogen chloride, preferably less than 1 ppm hydrogen chloride, and a stripped vapor phase.
Preferably, the stripping step includes the step of allowing the first condensed phase to come to room temperature, and then contacting it with helium at a pressure ranging from about 20 psig to about 30 psig [from about 240 kPa to about 440 kPa].
Also, preferred are processes in accordance with this aspect wherein the stripped vapor phase is routed to the secondary condenser to recover residual BCl3, and processes wherein the stream containing only traces of BCl3 from the secondary condenser is routed to a scrubber to remove residual traces of BCl3, along with HCl impurity and introduce a gaseous stream containing the inert gas and light impurities which are discharged to the atmosphere.
Further preferred processes in accordance with this aspect are those wherein the higher purity first condensed phase is transferred to a product container using ultra-high purity inert gas, preferably helium and without any other pumping or vacuum means.
A third aspect of the invention is a process for producing ultra-high purity boron trichloride in condensed phase from a lower purity boron trichloride condensed phase having phosgene impurity, the process comprising injecting an inert gas, preferably helium, into a container of lower purity BCl3 liquid having phosgene impurity to produce a vapor stream comprising BCl3, inert gas, and phosgene; decomposing a major portion of the phosgene in the BCl3, inert gas, phosgene vapor stream by heating to a first temperature to form a first intermediate vapor stream comprising BCl3, inert gas, carbon monoxide, chlorine and less than 10 ppm phosgene; adsorbing a major portion of the chlorine in the first intermediate vapor stream at a temperature lower than the first temperature using a solid adsorbent material, thereby producing the BCl3/inert gas vapor stream having less than 10 ppm phosgene and less than 10 ppm Cl2; routing said BCl3/inert gas vapor stream having less than about 10 ppm phosgene and less than 10 ppm Cl2 to a condenser; condensing a first vapor stream in the condenser, the first vapor comprising a major portion of BCl3 and a minor portion of HCl, inert gas, and light impurities to form a first condensed phase comprising BCl3 and a second vapor comprising the inert gas, residual BCl3, and light impurities; routing the second vapor stream to a secondary condenser, thus forming a gaseous stream containing only traces of (preferably less than about 10 ppm) BCl3 and a second condensed phase comprising BCl3; and routing the first condensed phase to a stripper (or using the secondary condenser itself at a more optimal temperature) wherein inert gas (preferably ultra-pure helium) is used to strip molecules having a vapor pressure greater than BCl3 from the first condensed phase to produce a higher purity first condensed phase having less than 50 ppm HCl, preferably less than 1 ppm HCl, and a stripped vapor phase.
A fourth aspect of the invention is a process for increasing the condensed phase production of BCl3 having less than about 10 ppm phosgene, less than about 10 ppm chlorine, less than about 10 ppm each of light impurities, and less than about 10 ppm HCl, the process comprising the steps of: introducing an inert gas selected from the group consisting of helium, argon, neon, xenon, krypton, and mixtures thereof into a container having condensed BCl3 therein, the condensed BCl3 having therein a minor portion of phosgene impurity; converting a major portion of the phosgene in the condensed BCl3 to carbon monoxide and chlorine by increasing temperature of the condensed BCl3; decreasing the temperature of the stream and removing the chlorine by adsorption and the carbon monoxide by stripping with an inert gas selected from the group consisting of helium, argon, xenon, krypton, neon, and mixtures thereof (preferably helium); and using the inert gas to transfer the BCl3 product to a product container.
In accordance with the present invention, several of the problems encountered in the prior art methods are overcome in the processes and apparatus of the present invention. By use of the inventive purification process technology, all significant impurities of interest in BCl3 for such high purity applications as semiconductor and fiber optic manufacturing are removed in the inventive processes such that a low purity boron trichloride now can be purified into an ultra-pure product with a purity of 99.9995% or higher (on a helium-free basis), or higher required for certain semiconductor and fiber optic manufacturing. The inventive processes and apparatus are preferably designed so as to minimize capital investment costs and to improve reliability. In addition, environmental emission is minimal, thereby reducing exhaust abatement requirements and increasing product yield. The inventive chemical process technology is composed of several different functional chemical processes or operating units as listed in the following:
Injecting an inert gas, preferably helium, into a source container of lower purity BCl3 liquid and extract the vapor out the container;
Using a functional catalyst such as activated carbon to thermally decompose phosgene at elevated temperature;
Using an adsorbent such as activated carbon to remove remaining chlorine at 50xc2x0 C. or lower;
Condensing BCl3 vapor which has substantially phosgene and chlorine than the source BCl3;
Using an inert gas to strip the BCl3 liquid to remove carbon monoxide, carbon dioxide, hydrogen chloride, nitrogen, oxygen and other lighter gas impurities that may be associated with lower purity BCl3 at the beginning, and/or generated during phosgene and chlorine removing processes upstream.
Transfilling the final BCl3 product from the inventive system into the product storage container using inert gas pressure and no other pumping or vacuum means.
It has been demonstrated that the inventive process technology is fully capable of producing an ultra-pure BCl3 product due to the following important new features.
Activated carbon is a particularly preferred material for the catalytic and adsorption steps, used both at high and low temperatures in such a way as to decompose phosgene and adsorb chlorine byproduct, respectively. One aspect that is surprising and unexpected in the present invention is that the carbon monoxide and chlorine byproducts of phosgene decomposition can be introduced into a lower temperature carbon bed without reformation of phosgene under the process conditions presented. The preferred activated carbon material was found to be fully regenerable to chlorine adsorption without degradation inactivity from BCl3. The preferred activated carbon catalyst which decomposes phosgene has shown the function of a catalyst at the elevated temperature, and therefore, the carbon can be continuously used without addressing the concern of saturation and regeneration.
An ultra-dry inert gas such as helium is employed in the inventive process technology which overcomes the problem of BCl3""s low vapor pressure, and the inert gas can drag BCl3 vapor out of the low purity container and carry the vapor through different purification process units. As a result, this process totally eliminates the requirement of heating the lower purity BCl3 liquid in order to provide enough vapor pressure penetrating each production process unit and of maintaining an isothermal operating condition in order to avoid the vapor condensation where the recondensation is not desired.
Further, the BCl3 purification processes and systems of the present invention do not require any mechanical devices either to transfer the low purity BCl3 into the purification system, or to transfill the final high purity product BCl3 from the inventive system into a storage container. The potential contamination on the final high purity product BCl3 by mechanical transfer means is therefore preferably eliminated, and consequently, the inventive processes and systems also operate more dependably and reliably because no mechanical component is involved in the transfer process.
In addition, the inventive processes and systems are able to run the vapor condensation and the liquid stripping separately, or simultaneously. Each chemical process unit operation of the inventive processes is preferably connected sequentially and the impurities removal operating is preferably continuously. The operating process minimizes potential air contamination and effects thereof because the entire process can be done without breaking down the system except changing the low purity and product containers. Besides, the production processes of the invention are very economical due to the product recovery from the process being 99.99% or higher within the secondary condenser, and consequently, this process technology is environmentally nonintrusive because the product is almost totally recovered with remaining trace BCl3 and HCl impurity easily removed by conventional scrubber technology.